The invention relates to a tungsten-rhenium filament with increased re-crystallization temperature. The invention also relates to a method for manufacturing such a rhenium-tungsten filament and a halogen incandescent lamp comprising the tungsten-rhenium filament.
Tungsten filaments for incandescent lamps are well known in the art. In most applications, the filaments are made of a wire which is wound into a coil. Coil dimensions determine not only the achievable light output of the lamp, but also the optical properties of the light beam emerging from the optical projector system of the lamp. Such projector systems are found, among others, in headlights of automobiles. Lamps with small filaments have better optical parameters and allow the formation of a well-defined projected beam, even with small-sized projecting optics.
Therefore, the coils with extremely small external dimensions are being produced for automotive lamps. The small external dimensions means that the inner diameter of the coils are also small, in the order of the wire diameter. The inner diameter of the coil largely corresponds to the diameter of the mandrel on which the filament is wound during manufacturing of the coil. The ratio of the diameter of the mandrel to the wire diameter is termed as the mandrel ratio. In this manner, coils with a small inner diameter will also have a small mandrel ratio. Since the filament wire diameter also has a practical lower limit, filaments with small mandrel ratio are necessary for achieving the best possible light efficiency.
During the filament production, the coiled filaments are annealed (heat treated to preserve the shape of the filament). This annealing serves to enable the assembly of the filaments on an automated mounting machine without breakage. During the annealing of the coil, a part of the coils made of wires with known tungsten-AKS composition tend to re-crystallize at least partly, and mainly on the compressed side of the coil. This partial re-crystallization significantly increases the probability that the coil will break. This leads to the failure of the lamp in a short time. As for these lamps the allowed defect rate is critical for marketability, a high defect rate cannot be tolerated.
In some special light sources, which provide outstanding optical parameters, the required parameters may be obtained only with coils having a very small mandrel ratio, in the order of 2 to 1.5, or even lower. This extreme mandrel ratio may cause a decrease of the re-crystallization temperature of the filament material. The exact physical mechanism of this effect is not known precisely. The decrease of the starting temperature of the re-crystallization process may be as large so that the initial re-crystallization temperature will fall in the temperature range of the annealing treatments used during the coil production. As a result, the re-crystallization process starts too early, already in the annealing phase, and thereby increases the mounting, shipping and installation defects, and thus impairing the production yield and reliability of the lamps. This significant decrease of the re-crystallization temperature may amount to 500 600xc2x0 C. on the inner parts of the coil which must endure the largest shaping tension or shaping stress.
In order to improve mechanical properties of the filaments, it has been suggested to include small amounts of rhenium in the tungsten. Typically, 1-3% by weight of rhenium is added. For example, UK Patent No. 1,053,020 teaches the addition of rhenium between 0.1-7% by weight, preferably 3% by weight. The improvement of the filament is achieved by promoting the formation of elongated grains in the tungsten, as it undergoes a re-crystallization during the lifetime of the lamp. The problem of decreased re-crystallization temperature is not recognized. The grain formation is also supported by grain shaping additives, as aluminum, potassium and silicon, commonly known as AKS.
Further, U.S. Pat. No. 5,072,147 suggests the use of tungsten filaments that are largely re-crystallized and have a grain structure with elongated interlocking grains. In order to quantify the quality of the grains, it is suggested to use the so-called grain shape parameter which is based partly on the value of the Grain Aspect Ratio (GAR). U.S. Pat. No. 5,072,147 stresses the importance of achieving a large value of the GAR because it is seen as a key factor for the so-called non-sag property of the filament. Again, no mention is made of the lower limit of the re-crystallization temperature.
U.S. Pat. No. 6,066,019 also mentions the use of a tungsten-rhenium filament which is re-crystallized before the lamp is actually used. This is necessary because the filament need to be mechanically supported during the re-crystallization. The re-crystallization temperature is above 2600xc2x0 C., in a relatively narrow temperature range. The problem of the decreased re-crystallization temperature in the strongly bent parts of the coil is not mentioned. On the contrary, the heat treatment method of the U.S. Pat. No. 6,066,019 inherently presumes a relatively uniform re-crystallization temperature range in the whole filament in which all parts of the filament start re-crystallizing only above a well-defined temperature.
U.S. Pat. No. 4,413,205 also suggests the use of rhenium for improving the properties of tungsten, but not for improving the grain structure or for modifying the re-crystallization temperature of the filament. Instead, the surface of the integral conductors is sought to be improved against the attacks of bromine. The suggested composition contains at least 0.1%, but preferably between 1-3% by weight of rhenium.
While the use of the AKS dopants and the use of rhenium in tungsten is well known for the filaments of incandescent lamps, the use of AKS by itself provides no solution to the problem of decreased re-crystallization temperature. The addition of AKS is mostly used to facilitate the grain forming process. However, with increasing color temperatures being typical for high-power automotive lamps, particularly with filaments that have operating temperatures above 2800xc2x0 K., an increased tendency of void formation on the grain boundaries is observed. These voids weaken the grain structure and accelerate the filament degrading process. The formation of the voids is attributed to the potassium. The addition of rhenium improves the grain structure of the filament and thereby compensates the negative effect of the potassium, at least partly. It was believed that the addition of at least 1% by weight rhenium is necessary to compensate for the void forming effect in filaments operating at high temperatures.
It was observed that the grain structure and thereby the mechanical properties improve with higher amounts of rhenium, but even small amounts (as little as 1%) increase the temperature necessary for the complete re-crystallization for tungsten filaments above the critical value of 2600-2700xc2x0 K. With presently available mass production technology, the filaments may be heated up to approx. 2750xc2x0 K. during the re-crystallization. Raising the final re-crystallization temperature above this value would significantly increase the cost of the filament manufacturing.
Therefore, there is a need for a tungsten-rhenium filament having an initial re-crystallization temperature above the annealing temperature of the filament, which at the same time has optimum grain structure, and which may be manufactured economically.
In an embodiment of a first aspect of the present invention, there is provided a filament made of a tungsten-rhenium alloy wire. The wire has a re-crystallization temperature above 2000xc2x0 C. The filament wire comprises AKS additive. The wire material has a potassium content between 80-110 ppm, and a rhenium content of 0.05-0.19% by weight.
In an embodiment of a second aspect of the invention, a method for manufacturing the rhenium-tungsten filament wire comprises the following steps. An AKS doped tungsten-rhenium alloy powder is prepared, preferably by blending together AKS doped tungsten powder and rhenium powder. The blended alloy powder has a rhenium content of 0.05-0.19% by weight and a potassium content between 80-110 ppm. The alloy powder is pressed and presintered. Thereafter, the alloy powder is sintered with direct current. A filament wire with a metastable crystal structure is formed of the sintered alloy. The wire is wound on a mandrel, and it is annealed on the mandrel while in the metastable crystal structure, and the annealing is done on a temperature below 2000xc2x0 C. (approx. 2300xc2x0 K.). The filament wire is re-crystallization at a temperature above the re-crystallization temperature to achieve a stable crystal structure.
The tungsten wire produced on the basis of the method results in improved filament stability because the re-crystallization of the coiled filament starts at a significantly higher temperature, even with extremely small mandrel ratio.
In another embodiment of a further aspect of the invention, a halogen incandescent lamp comprises an envelope enclosing a tungsten-rhenium filament. The filament comprises an AKS additive. The potassium content of the filament is between 80-110 ppm in the filament, and the filament has a rhenium content of 0.05-0.19% by weight.